If you are getting the "wrong signature length" error when
running the OpenSSL "req -verify" command,
the CSR you are trying to verify has invalid digital signature.

There 2 main possibilities for a CSR to have invalid digital signature:

The CSR has been modified by someone.

The CSR has been signed by the wrong private key.

The first case is unlikely to happen, if the CSR is generated by you.
Unless someone hacked your computer.

The second case may happen, if you convert a certificate to CSR and
sign it with the wrong private key.

For example, you find that your server certificate is about to expire.
So you download it and use the "x509 -x509toreq" command to convert it
to a new CSR. This ensures that the new CSR has exactly the same
DN (Distinguished Name) fields as the existing certificate.

But if you are not able to find the private key that matches the public
key in the certificate, and use another private key with the "x509 -x509toreq",
you will end up with an invalid CSR of case 2 listed above.

In the test below, we are going to download linkedin.com certificate
and convert it to a new CSR using my own private to sign.
Of course, the new CSR will be no good.

The error message in the "req -verify" command output
confirms that twitter.csr is generated by the "x509 -x509toreq" command is invalid.
Signing someone else's public key in the CSR with your private key is not acceptable.